The Maunder Minimum lasted for approximately 70 years from about 1645 to 1715, and was marked by bitter cold, widespread crop failures, and severe human privation. There has been increasing evidence in recent years to support this supposition that global warming is linked with solar activity. In 2011, three papers suggested the Earth could be heading for a ‘little ice age’ as solar activity drops once again.

Since the 1940s and up to 10 years ago we have had the highest solar activity in 1000 years. The last time we had solar activity that high was when we had the Medieval Warm Period from year 1000 to around 1300. … Historically there has been a close connection between solar activity and temperature for the last 1000 years. Therefore the sun’s activity will also have influence the coming many years. … The unusual thing right now is that sun’s activity is decreasing while there’s a great increase in atmospheric CO2. For that reason the question is how much the earth will cool in a time of decreasing solar activity. … The development is beautifully consistent with a cooling effect of the solar activity in the same period. This could mean that the temperature will not rise for the next 30 years or maybe begin to decrease.” -Danish astrophysicist Henrik Svensmark

"Theoretically this relationship has been known a long time. The parallelism between global temperature and solar activity over the past 1000 years appears to confirm it." -German online daily Die Welt

<2267> Wilson: Although I agree that GHGs are important in the 19th/20th century (especially since the 1970s), if the weighting of solar forcing was stronger in the models, surely this would diminish the significance of GHGs. [...] it seems to me that by weighting the solar irradiance more strongly in the models, then much of the 19th to mid 20th century warming can be explained from the sun alone.

Now, an international team of researchers led by Ilya Usoskin of the Sodankylä Geophysical Observatory at the University of Oulu, Finland, may have the answer. They examined meteorites that had fallen to Earth over the past 240 years. By analyzing the amount of titanium 44, a radioactive isotope, the team found a significant increase in the Sun's radioactive output during the 20th century.